One value eg. 800 x 600, is a completely different value than the weight of your pic eg. 100k

800 x 600 is the height and width of your pic (in pixels), 100k (in bytes) is the weight.
To upload to SimV, the pic had to be 100k in WEIGHT or less. You can change this value for any pic you wish to upload, with a simple image processor like Irfanview, available here at the V.

So, you can have a pic 800 x 600 and save it as a jpeg at 100k, then re-save the same 800 x 600 pic as 50k for example. The image quality of the 50k pic would be inferior to the 100k pic. The trick is to size your pic to 800 x 600, a standard SimV size, then save it as close to 100k as you can with Irfanview to acheive the best possible quality.

You can find Irfanview here: http://www.simviation.com/fsutilities_gen3.htm about halfway down the page. Look it over and if need be, come back with more questions. We'll straighten you out sooner or later..