I think you should just have to edit the corresponding templates. To find out what template something is in, you can turn on the "Add template Names in HTML Comments" option (In the admin control panel, go to vBulletin options -> general settings). Then you can view the HTML source of your page and look at the comments. Sometimes it's annoying because lots of templates are nested and going backwards you find "end of template" "begin template" "end of template" "begin template" etc lots of times, in that case it might be easier to scroll to the top and see what the "top level" template is and check that one first.
To edit a template, go to Styles and Templates -> Style Manager then pick "Edit Templates" from the dropdown next to the style you want to edit, then choose the template from the drop-down. The list is organized hierarchically so you may have to search around. Or you can use the "Search in Templates" function with "Search Titles Only" to find one quickly (of course you can also use "Search Titles Only = no" to search the contents of the templates).
If you haven't seen it already, the manual is at
http://www.vbulletin.com/docs/html/ (make sure to choose your version from the drop-down because it defauts to a 4.0 version).
ETA: So that I don't mislead anyone in the future - you can change from images to text by editing the template for everything except "multiquote" - see this thread:
https://vborg.vbsupport.ru/showthread.php?t=224978