NOTE: This tutorial is very old. It was made in January 2007, and Photoshop has been updated numerous times. You might want to look for a tutorial that is more appropriate for the version you are using.
NOTE: The new maximum for attachments is 150KB. The new maximum dimensions for all embedded images is 1024 X 1024px.
Resizing Images Using Save For Web
Saving images to sizes that are appropriate for viewing on the web can sometimes be tricky. This tutorial discusses the process of saving your photo using Photoshop's Save For Web feature.
The POTN Image Posting Rules state that any image posted here must be no longer than 800 pixels on the longest side. Furthermore, if you use the Attach feature you're limited to 100KB. If you host your image on another site you can have a larger file size, but I'll stick with 100KB for this tutorial. (If you do host your images on another site please try to stay below 150 KB. It's not a rule, but larger images slow people down and it's almost never necessary to go above 150KB for an 800-pixel image.)
Resize The Image
The first thing you want to do is get the image so that it's 800 pixels on the longest side. The easiest way to do this is using Photoshop's Fit Image tool. In CS2 this can be found by clicking on File... Automate... Fit Image. A small box will pop up saying "Constrain Within" and listing 2 boxes, Width and Height. Simply type in "800" in both boxes and click OK. Your image will be re-sized to 800 pixels on the longest side.
If you don't have Fit Image then you can click on Image... Image Size. You'll get the following box:
On the bottom left make sure that all 3 boxes are checked. For the resampling type I always use Bicubic. Bicubic Sharper is good for resizing images smaller, but I never use it because I like to control the sharpening and Bicubic Sharper doesn't allow you to change the sharpening strength or type.
In the top boxes, under Pixel Dimensions, change the Width to 800. You'll see that the Height changes automatically. If the Height is larger than 800 simple change that to 800 and the Width will change automatically.
Ignore all the other boxes, no matter what they say. Trust me. Just click OK and your image will resize.
Sharpen The Image
Next you want to view at 100%, so click on View... Actual Pixels or hit Control-Alt-0 (zero).
At this time you want to sharpen the image to suit this size. I will not go into sharpening here - it's far too complicated for a quick tutorial.
Convert to sRGB
To make sure that the image's color display properly on the web you will want to convert to the sRGB colorspace. (If your working colorspace is already sRGB you can skip this.)
The process for doing this changed in Photoshop CS2, and it's been a while since I used older versions of PS, but...
In PS CS or older: Click on Image... Mode... Convert to Profile
In PS CS2: Click on Edit... Convert to Profile.
A box will pop up with some options.
The options that I almost always use are shown above:
* Destination Space: sRGB IEC61966-2.1
* Engine: Adobe
* Intent: Perceptual
* Use Black Point Compensation: Checked
* Use Dither: Unchecked
* Flattten Image: Checked
(These last two might be grayed out. Don't worry.)
Preview The Color Changes
Check and uncheck the Preview box a few times to see if any color change is occurring. You should not be able to detect any change - well, you might but it's almost always very slight.
The Save For Web Dialog
Next click on File... Save For Web.
Along the top are 4 tabs. Click on "2-Up" which will display the original image (on the left) next to the image that will be saved.
Near the top rght corner you'll have a box with choices of GIF, JPG, PNG, etc. Set it to JPG.
Below that uncheck both Progressive and ICC Profile.
To the right of that check Optimized
Now look at the image on the right, and look all the way at the bottom. You'll see something like:
As long as that number below "JPEG" is 100.00K or lower you're fine. But if it's larger, look back up near the top right corner, to the right of where you set JPG, you'll see a box called Quality and it will probably say "100" in it but will say something between 1 and 100.
Click the arrow to the right of the number and a slider will pop up.
Grab the triangle and slide it until the size is below 100.00k.
Compare The Changes
Now check out the image on the right, which is what it will look like once you save. Does it look OK? Compare this to the original on the left.
You can use the tools on the left, Hand and Zoom, to scroll around or zoom in. You'll see that both images scroll or zoom simultaneously allowing to to compare. It's a very good idea to zoom in to compare and important feature, like a subject's eyes. I never zoom in more than once, which puts you at 200%.
So how do they compare? If the right image looks OK, and you're below 100.00K then you can just hit Save in the top right corner. But if the image doesn't look so great you may need to start all over again with a smaller image. Go back to the beginning of this tutorial, but instead of resizing to 800 pixels try 700 pixels. It may even take a few tries to get the image as small as possible while retaining quality.
Save The New Image
So once everything is set - 100.00K and the image on the right looks good - click on the Save button in the top right corner. You'll be prompted for a file - just be sure not to over-write your original! I save my web images in a separate directory to make sure that these version are kept separate from the originals.
This process should get you a fine-looking image at 800 pixels and 100 KB. Here's an example - 800x533 pixels and 99.62 KB:
I hope this helps some folks. Enjoy!
Some Useful Links
POTN Image Posting Rules
Tutorial: How to attach photos to a post
Tutorial: Where to host my photos