Supported Image formats for image in visible signatures

[zeridon]

What are the supported image formats for adding images to visible signatures.

By Process of Elimination PNG is supported but some explicit list would be greatly appreciated.

Additionally if possible support for SVG/PDF would be greatly appreciated.

[kwart]

These were added to the docs: GIF, JPEG, JPEG2000, PNG, WMF, BMP, and TIFF.

Let's review once the #185 is merged.