Test of larger image, 960 x 1280 pixels, and a 99.3 KB filesize.
It seems to me that the Forum restriction on image size is not the dimension of image in pixels or inches or centimeters or whatever. Rather is the size of the file in KBs when it is stored on your hard drive, or the Forum's server/storage.
The filesize of an image on your hard drive is a function of many things. The dimensions of the image. The setup of the camera, including megapixels, zoom, image format, etc. The type of file, jpg, gif, bmp, tif, raw, etc. Bit depth of the color used to render the image, 16 bits, 32 bits, etc. The 'save as' choices for some image formats, especially jpg.
A jpg image is saved in a compressed format. Most image programs give you some control over the amount of compression. For instance, with an image of 400 x 600 pixels, a 1 MB image filesize, and a compression ratio of 1:1, the file will still be 1 MB after you save it.
Use the same image, but set the jpg compression to 2:1, or 50%, and your image will still be 400 x 600 pixels, but the filesize will be 500 KBs after you save it. The other 500 KBs are thrown away by the jpg algorithms.
With the original 1:1 compression, you may have been able to enlarge the image a bit, and not introduced any artifacts like blockyness, or loss of focus. But with the 2:1 compression, your ability to enlarge is greatly reduced because 50% of the images original data is gone.
