In terms of Clip Art, see Use of Microsoft Copyrighted Content and Microsoft Services Agreement, particularly 8.1 in the latter. So long as you can meet the terms and conditions outlined particularly in that clause (as well as the rest of the Microsoft Services Agreement - i.e., have a licensed copy of the software, etc.), it is permissible to post your slides with Clip Art, particularly when using a secure repository like Nexus.
There is the Open Clip Art Library that is searchable along with other Creative Commons media repositories at http://search.creativecommons.org/ - CC materials still come with copyright, but the terms of use for those materials are often much less restrictive (and you may want to pursue this, especially if you're considering posting material more openly).
For other images, consider https://copyright.uwinnipeg.ca/format-specific-guidelines/images.html You may use images under fair dealing for purposes of review, criticism, satire, parody, private study, research or education. If the images are used within the fair dealing purposes of criticism or review you may use them with proper attribution of the source and accompanied by notes which comment directly upon the images.
