Can you use images taken at different exposure times in western blot image analysis?
Generally no, you can't directly compare them and you can't easily compare across gels. I have done many hundreds of western blots, a large proportion of which were for quantitation, and in general quantitation on western blots is difficult (because of inconsistency run to run) and not highly quantitative - a 2-fold change is detectable and probably real, but a 10% change - unlikely to be real.
If you want a real example of how to compare and observe this (works with the Bio-Rad system or others where the transfer is in the same space) - run exactly the same amount of protein on two identical gels, place them both into the same transfer tank. Compare protein stain on the membranes after transfer. The one closest to the black terminal will have stronger staining = better transfer.
However, what you can do is convert the numbers you get into a relative expression on each of the blots and then use those numbers for comparison.
For example you compare expression of protein X to a normalizer (often B-Actin) and get fold-change data out of that, or you can compare Sample X untreated to treatments A, B, C and get data out that way.