GIMP (GNU Image Manipulation Programme), is an open source picture editing programme that works on many operating systems. It is a very powerful picture management programme, although a bit difficult to handle.
Irfan Skiljan developed a very light user-friendly picture editing programme that allows you to carry out basic adjustments functions to your pictures on the fly, like sharpening, colour management, stitching photos, together, etc. Don't forget to download the plug-ins http://www.irfanview.com/plugins.htm, as they can help you do some very useful tasks. The one I use most is the "Safe for Web" plug-in that compresses images to less than a quarter without reducing the number of pixels. There is also a OCR plugin, but that is rather useless.
When posting a picture on line, everybody can copy that picture and use it without paying tribute or money to you. To stop that from happening, you can place a watermark in your photo. I have tried to do so in Gimp, but that is quite cumbersome and slow. In Irfan View one can place a text, but I could not find a way to make it transparent, so the text is very dominant. Moreover, I could not find a way to make it diagonal. A diagonal watermark is very effective as it makes it impossible to use any part of the picture without showing the watermark. Now, Picture Shark apparently has been developed just to make water marks and it is fast, it allows for making diagonal texts and the watermark can be made semi transparent, thus softening the effect of the watermark and still rendering your picture useless for illegal printing.
VLC is a free and open source cross-platform multimedia ((video) player and framework that plays most multimedia files as well as DVD, Audio CD, VCD, and various streaming protocols. This is an absolute must have for anybody who likes to edit on his/her videos. Don't forget to also download the extensions and plug-ins: http://addons.videolan.org/index.php . You can manage colours and sharpness on the fly. You will enjoy this video editing programme.
Audacity is an absolute must for making sound with your presentations. Audacity is free, open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems
Audacity can record live audio through a microphone or mixer, or digitize recordings from cassette tapes, vinyl records, or minidiscs. With some sound cards, it can also capture streaming audio.
Import sound files, edit them, and combine them with other files or new recordings. Export your recordings in several common file formats.