How to do fast display of Images

I'm having a php page which displays a set of category names.These names come from the mysql database along with corresponding images. When a user moves his mouse over any of the category name the corresponding images gets displayed. All this works fine in my local system the same page after uploading and testing the images seems to appear after some large delay during mouseover effect, Once it gets displayed further mouse over events appear to be faster enough, during the first loading of the page the delay in the image appearance appears.... What could be the reason ?...Do i have to do in some other way... ? Help me over this Thanks in Advance Param