It likely has nothing to do with computer security but about image size or format eg .bmp or .jpg
It took me a while to figure out this myself, learning to post pictures took me a long time to figure out, I am still pretty much a computer dummy but slowly learning how to do things.
The avatar that goes with your name has pretty strict rules that vary from site to site on what size you can use.
Here the picture used for the avatar must 100 or less pixels both in height and width.
this free software program irfanview from
www.irfanview.com/ makes creating avatars easy, and fairly simple to use
Once you download and open the picture you want to use
hold down the left button on the mouse use the arrow to form a square on the part of the picture you want to use, if you make a mistake just click on the picture and you can start over with the square, once you are happy with the imagine inside the square , then click on the edit button and a drop down menu appears, click on "crop selection" and only the image inside the sqare you selected will appear, now click the image button from the drop down button select "resize/resample"
On the screen where it shows the size pick height or width whichever is the larger number and change it to 100 then press okay.
Click file select "save as" , and change the filename so as not to lose the original picture something maybe like "gra avatar" make sure the file type is "Jpg - jpeg files" by changing the filename you won't lose the original picture off you computer
Now that picture must be uploaded to the web, there are a lot of photo sharing site but most will not allow you to use on other sites unless you pay for upgraded service ,
www.photobucket.com will allow for free , so it makes for a good site to use to upload to, so when you upload find the picture called "gra avatar.jpg" once it is uploaded you can copy the url from the box under the picture by clicking on the box that url then right-click to select copy and then here on this site right click where it asks for the url and select paste and you can add your picture to your profile.
when you want to use pictures inside your replies you the IMG under the picture on Photobucket
and copy and paste in the same manner , right into the post you are replying to, you do not need to use any of the tag buttons at the top of the reply box, you do not need to resize these photos that are used in your posts.
I sometimes will still use irfanview to edit my photos just to crop or enhance colours and alter sizes if I feel cropping has made them too small
eg original picture
then cropped and made larger using irfanview