Can I make my Avatar from postimages? I have it uploaded on my gallery. I can't resize it though.

You can't upload an avatar to wolfplay using postimg, no.

To resize an image, you'll want to use something like this website that will allow you to crop/resize your image to 300 by 300 pixels

From there, you'll be able to upload your avatar to wolfplay on the account page under 'assign pack avatar'. Make sure to read all the information on the assign pack avatar drop down and ensure your avatar is properly credited. Please let me know if you have any further questions or are still confused.

PostImages is no longer a site we allow to be posted from due to innapropriate advertisements. OLD art will be allowed to stay there as long as there aren't any problems, however ALL new art will need to be uploaded on safer image sites.

What other websites can I upload from?
Im an artist, no idea what to do! :(

I've been using Filegarnest for over a year lol. Imgur is also allowed so far, so is Gifyu. But here's other sites in a list if you don't like those.