Nope.
A signature may contain one or more images but they must fit within a box with maximum dimensions of 400 x 75 pixels (width x height) and with a maximum (combined) file size of 20k.
Images containing mostly very large words or animated .gifs will not be permitted. You may also add 1 consecutive lines of normal or smaller text directly above or underneath the image with no spaces inbetween image and text.
If no images are used then up to 4 consecutive lines of normal or smaller text is permitted. Do not pad out your text lines with blanks lines. A line of text is defined as having a maximum length of 400 pixels.