Here, for use by any forum member, are five general tips to solve the problem of a too-large image that you want to attach to a post. Remember that the magic number is 102,400. You want a file no larger than 102,400 bytes because that's the forum's maximum acceptable attachment size. Use Get Info in the Finder to check a file's size.
1. If you want to attach an image that shows more than necessary (such as a screenshot where only part of the screen is of interest, or a photo that is not cropped to the portion of interest), import it into iPhoto (or another image editor), crop it, and Save or Export the result.
2. To capture a portion of the screen, press Shift-Command-4. The cursor will change to a bullseye. Use it to drag a rectangle anywhere on the screen. You'll get a file named Picture n.pdf (where n is the next available integer). Double-click it to open it in the Preview application and then use File->Export to save it as a JPEG or PNG file.
3. To make an image smaller, you can open it in Preview, use the File->Export menu choice to save it in JPEG or PNG format, but set options to make it smaller by clicking the Options... button before you click Save. You can experiment with the format and options to find a small-enough result that still looks nice:
3a. For JPEG, you can pick Least or Low quality to make a smaller file. (The Target size feature would be perfect if it worked, but it rarely works for me.)
3b. For PNG, you can set the Depth to 256 colors, but you'll have to make sure it still looks good enough afterwards. If it is a cartoon-style graphic, it'll probably be OK. If it is a photograph, it'll probably won't be OK.
For each JPEG or PNG file you create, re-open it in Preview to see if it still looks OK.
4. You can use iPhoto to scale an image. Import the image into iPhoto, select it, and choose File->Export File. In the Export Photos dialog box, select the File Export section at the top, select JPG or PNG as the Format, and use the "Scale images no larger than" radio button to pick a width or height smaller than the full image. This will scale the image. Making the height or width 1/2 as big will make the image file 1/4 as big, so you may not have to scale it a lot. By experimenting, you can find the scaling factor that gets you down to the file size limit.
Note that JPEG and JPG mean the same thing. The real name of the format is JPEG, but many programs (including iPhoto) call it JPG because people are so used to using three-letter extensions, a holdover from older operating systems.
If you have another image-manipulation program (e.g., Photoshop or software that came with your scanner or camera) , you can use it to scale images too.
5. The last resort: If you have access to a web server (like your own .mac site or a friend who won't mind posting the image for you), then instead of attaching the image to a MacRumors post, post it somewhere on the Internet and put its URL inside [ IMG ] ... [ / IMG ] tags in your post.
Now, how do these tips apply to your case, wdlove? I expect that tips 2 and 3a apply to your situation. It sounds like you don't really need to capture the whole screen to show us the problem you are describing. If the dock is at the bottom of your screen (I forgot to ask about this) and you see an odd color change in the bottom 1/3 of the screen, then you could show us the problem if you capture a thin-to-medium vertical strip from the top to bottom of the screen. In other words, it doesn't need to be the whole width of the screen to show us the problem. If it was, say, 20% of the screen width instead of 100%, it'll be that much smaller. If it's still too big, use tip 3a. If that doesn't work, use tip 4.
