Email signature appears as an attachment

I've been trying for a long time to solve this matter, but I do not seem to be able to do that.

I've made myself a signature for my email and this signature includes a png picture underneath the text, and I found out that every time I send someone an email, it turns out the png in my email signature is appearing as an attachment.

So does anyone knows how to stick the picture to the signature and make appear as a single signature?

Thank you.