There are several ways to do this. (1) Inquisit can present html-pages as instructions. Just write your instructions in MS Word (for example) and put the picture in (as you would with any normal text document). Finally save the document as webpage (MS Word can do this as can most other word processors) and include with your Inquisit script using the <htmlpage> element. (2) You could also use a normal Inquisit <trial> element to present your instructions (i.e. text and pictures). Just set it up like any other trial in Inquisit and run it before the other experimental trials. You can also turn off data recording for your instructions trial by setting '/ recorddata = false' for said trial.
Hope this helps,
~Dave