I was having a problem with that to, can upload more then you can add to listing, so I now use a image software to help solve this when I need more then 16 images.
So I will put more then 1 image in a image, like if I have an item such as clothing and i want to show close ups of an items this sometimes will take alot of images, so i put what i can in same pic .... hard to explain but you can look at this link where I had to do that, first link at bottom of this post
It dont hurt to just put 2 or 3 or 4 images in same image box with a image program. I use photoshop for all my image editing etc i just select new under file in photoshop and put size box i want and add what images in it i want,
First i resize them usually i put mine 500 for width and height will go auto by having the constrain box checked or images will not look right, then i copy and paste what images i want to new box side by side maybe if i use say 6 images put 2 on top then 2 under those 2 and another 2 under those 2 or however you want so long its a even square thats why its important images all be same size if possible to look nice,
and then crop around the images i just addded which is now just 1 iamge and then again make it 500 x whatever the constrain option puts height will be and save it...
well thats how I do until I find another way sounds, like alot but really its not, its much easier to do then to explain here and with their supersize when you click the image it goes bigger so its looks ok. Take a look
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=170042613418