.pbt is a file extension for a theme, not an image. Don't attempt to change the file extension of an image to .pbt ... no good will ever come from that.
.png is a valid extension for an image you might choose to use as a background image. You should have no problem using such an image (unless the filesize exceeds proboards maximums).
Through your Admin Panel, navigate: Admin Home> Themes> Advanced Styles & CSS> stay in the Visual Editor tab. Under the heading Forum, click on Background. That section will expand. To the right of Background Image you'll see a box with text in it. To the right of that, you'll see this icon: . Click that icon and a popup will appear which will allow you to select an image on your computer which you can upload to use as the forum background.
I'm not an expert, I'm one of your peers. A forum owner, just like you. Sharing knowledge builds a better community. Proud Member of Guitarnutz2