You can store information in the array as to the size and location of an image in that array. For example, the first section of the array can be treated as a file header, wherein you store data about the number of images, their size and location within the array, etc. As data changes during program run, this "header" can be updated accordingly.
BTW, for using the temporary PSET during program run:
TempCOLOR = POINT(x, y) 'bottom-right corner of area to GET
PSET(x, y), 15 'temporary PSET
'determine array size
PSET(x, y), TempCOLOR
Although, I was thinking that if you're going to be establishing array size during program run, you may want to use QBasic's formula. It's a bit more complicated than the empirical method, but would be simpler in that it would only involve a calculation. To check it out, go to QBasic Help/Index/GET(graphics) then click (Screen Image Arrays and Compatibility).