From the sound of it, best match rankings might be affected if image sizes are smaller.
I've added the requirements below for anyone who is interested and will post the link to the full details below that.
quote:New picture requirements
In 2013, requirements for pictures will be updated to ensure quality imagery on the site.
All sellers will be required to upload at least one picture for each listing – Remember, you can now upload up to 12 pictures for free! Adding high quality pictures to your listing helps you increase sales, gives your buyers a more accurate picture of what to expect, and distinguishes your listing from others of its kind
Minimum picture size will be enforced – All pictures will need to be at least 500 pixels on the longest side. However, eBay recommends 1600 pixels on the longest side so you can take full advantage of eBay picture services including Supersize.
1600 megapixel pictures can be taken with an average 2 MP camera or better
Note: Picture size doesn’t slow down picture loading speed for buyers
Other picture requirements to be introduced next year may include the removal of text and borders. Small translucent watermarks in the bottom-right corner of images will still be allowed.
ebay changes for 2013