I find the html I can use in the Text Widjits to be slightly different than the html I can use in each theme...
This will give you a line (just take out the x, and "50" is the length in pixels)
<xhr width="50" align="center" />
<xbr> is a soft return or break, which (style-wise) is more pleasing than a hard return or <xp>.
you can use all of the minor stuff, like bold/center/italics/font color/font type/photo links ...of course you have to make sure you use the American spellings (color, gray, center). Each text widjit can have as many photos or text as you want, just make sure the photo fits. Each theme has different sized sidebars... the size which fits the one I'm using is 225px. Some themes get pretty small...