There is no requirement to do so, but you may want to resize your pictures before posting. Many years ago, camera’s didn’t take mega pixel pictures, but today most do, even camera’s on phones. The problem with mega pixel pictures is that the file size on them is very large, which can be slow to download for those that don’t have speed internet, or for those that use a cellular data plan with data limits. 400px X 600px (or 600X400) is the largest optimum size. There are peeps that try to help, but when the pics are too big, and slow to load they may back out of a thread. The peep(s) who back out may be the very one that can help you with your item.
The attachment option at the bottom of the page only allows one attachment to each post and you have to reply to add another pic. The attachment option also does not allow you to view the picture before you post to see if it is an appropriate size.
The alternative, and IMHO better, way to add pics is to use the tree icon in the area above where you enter text. It allows you to add multiple images to a single post from both your computer as well as from other websites. If you select "Large" as the image size, the picture will resize to a good size, and it will also reduce the number of bytes in the file.
This thread may help you understand how to post using the tree icon
https://community.ebay.com/t5/Pottery-Glass-Porcelain/OT-How-to-Post-Photos-Using-the-Tree-Icon-the-...
