Google has a lot of ways to check if your site is relevant to the keywords that users are searching with. One of these ways is to check the URL path of images. For example, when a page would be www.example.com/candle.jpg, the image "candle" would appear in google images and also Google recognises this page "example.com" does something with candles, and therefore the pageranking will increase on the word "candle".
When you upload an image to your Weebly page, Weebly changes the name of the image you're uploading. So when I upload an image with the name "Candle.jpg", Weebly changes this to for eaxmple "2373015_orig.jpg". This is not the most optimal thing for SEO. Allthough it's possible to change the ALT tag of the image (you can make ALT="Candle"), it would be much better if the filename itself would be candle.jpg as well. This would realy fill google images with your images.
So for the best ranking in search engines it is absolutely necessary to be able to control the name of the images. I sugest Weebly is making it possible to control the filename for a better SEO!