Check this out about posting an image in the forums
http://forum.avast.com/index.php?topic=8982.0. You must first have captured the image and saved it as an image file format as I mentioned earlier, you can't just copy and past it into the post, it first needs to be converted from the clipboard data to an image file.
If you are going to send emails with image attachments there is nothing to stop you stopping the outbound email scan (see image) or you could stop scanning outbound email completely, not something I would recommend.
Have you tried what I suggested with the no spaces in the image file name ?
We ask questions and offer suggestions to eliminate possibilities and to try and give as detailed information as possible. If you don't answer the questions or try the suggestions and let us know the outcome, we are in the dark and keep guessing, which doesn't help us or you.
Short answer to switching to your ISPs AV solution, NO. You don't even know what there AV is or if it would provide the same level of protection or flexibility as avast and to switch for what is a pain but not a security issue I feel is a poor choice, the decision is entirely yours though it is your system. You have also got to consider what happens if you experience a problem with that AV, who will support you then.