Sure...
1.) Upload your image to another site like Facebook, Twitter, Tumblr, Imgur, DeviantArt, any site.
2.) Right click your uploaded image from another site and copy the image location url.
3.) In the Tapastic forums click the Reply button on the forums to make a comment on a thread.
4.) type in this code into the text box
<img src="">
5.) between the quotation marks in the code, paste in your image location url that you've uploaded to another site.
For example it'll look something like this:
<img src="https://d30womf5coomej.cloudfront.net/ua/b9/bb6c0f14-351c-4186-9722-0e0c134675d1.jpg">
6.) Click the Reply button when you've finished your comment to a thread.
Image with be there like this: