The image on my profile always gets truncated. I tried different images and different sizes, but they all get truncated.
I could not find any image size information in the Stack Overflow help file.
It doesn't look to me like it's being truncated. Rather, it looks like it's getting compressed (or squished, if you prefer).
Truncated would mean that some portion of it was getting cut off…like this:
Instead, it's being automatically compressed to fit into the space available for profile pictures:
You can generate the same effect yourself in any image editing program by reducing the horizontal dimension without reducing the vertical dimension. In Microsoft Paint, click "Resize", uncheck "Maintain aspect ratio", and make the horizontal size smaller.
This is a hint—the aspect ratio of your original image does not match that used for profile pics. In other words, it's a rectangle, but profile pictures are squares.
The size of the large image displayed on your profile page is something like 128x128 pixels, but the actual size of the original image is not particularly important. The image is resized automatically in different parts of the UI, depending on the space available and the focus of the content. If you upload a square image, it will look right.
And like others have suggested, now is a good time to consider changing your profile picture to something a bit more appropriate. Look for images that are approximately square. Like this one: