On a windows PC, right click on the pictures icon, select "open with" and choose Paint, which is the native Windows Image Editor. With the image opened in Paint, click on the Home Tab, and click Resize. In the menu that opens, select Pixels rather than Percentage and enter the desired number of pixels for the horizontal or the vertical axis. If maintain aspect ratio is ticked, whichever axis you define Paint will also change the other to ensure the picture stays in proportion.
It may be necessary to crop the image to the forum requirements. You can do this by trimming the image to suit using the rectangular selection tool, then hitting crop. Now save the file with a new name so that you still have a copy of the unedited version. This procedure works with both Windows 7 and Windows 8.

On a Mac computer, open the image with the preview programme then select Adjust Size from the tools menu to reduce the image dimensions. Verify the new size by highlighting the file and pressing Command and "I" keys together.