Can I ask why you use Incredimail? from its web page its nothing I'd go near with a 100' barge pole..... what wrong with good old basic email, plain text with attachments as required, you stand far less chance of "catching" something without all these fancy adornments!
it looks like its changing the file in transit if its changing the name. ATT I'm guessing is that it thinks its a unformatted atttachment rather than knowing its a jpg/image hence it opening in Notepad and not Picasa as images should. try saving the attachment to disk and renaming it blah.jpg and then open it from there. Unfortunately Windows "hides" the true file extention by default for some silly reason so you need to change your folder view settings to show file extentions or it will just end up blah.jpg.txt which will still open with notepad ( as notreapd is the default for .txt files.)