NB: This tutorial is for GIMP, though similar steps will be taken for other image manipulation programs.
Step 1: Find a photo. Photo manipulation is outside the scope of this tutorial, but you can start by checking out my tutorial on color correction.
Step 2: Make sure you have the Selection tool selected, and then use the drop-down menu to make it a "Fixed Aspect Ratio"
Fixed Aspect Ratio
Step 3: Set the Aspect Ratio to the size required by your Forum software (in this case it is 500x200).
500 by 200
Step 4: Select the area you want, and then choose Image->Crop Image.
Step 5: Now go to Image->Scale Image.
Step 6: Make the image the proper size (in this case, 500 by 200 pixels).
Step 7: Now go to File->Save As.
Step 8: Name your file, and make sure you select the JPEG File Type before you Save.
Step 9: Choose your quality setting. 85 is just fine*.
Step 10: Go back to the Forums and upload the image from your computer.
*If you'll notice, for these forums my image must not only be within the 500x200 pixel restraints, it must also be no larger than 29.3KB. So, I actually had to open my image again (which was 31KB) and resave it at a quality of 60 so it would be small enough. You will likely have to do the same.
So, there you have it: How to make a signature image for your favorite forums.
Your Media Production Mentor