If you find an Open Textbook you want to use for your class, you can adopt it. You can simply provide the link in Nexus and all your students will be able to access it. If you do adopt a textbook, let Campus Manitoba know — it helps with the long term viability of the project to know there has been uptake!
Regardless of platform, there are a couple of basics to presenting and organizing content you should keep in mind.
The most common platform for hosting OER in Canada is Pressbooks, but some people use GitHub as well. If you are interested in using Pressbooks to create a professional textbook, please contact Brianne Selman.
Images you find "on the internet" are usually copyrighted, which means that you need to be careful about how you reuse them. However, there is an ever growing amount of images that have been made available with Creative Commons licenses, or other licenses that allow reuse. If you are going to use these images, familiarize yourself with the license types - CC-By, for example, just requires attribution of the creator, but CC-SA requires that anything that reuses that work also be licensed with the same Creative Commons license.
The following resources are excellent for finding images that allow some reuse:
In addition to open images, Creative Commons has a very good list of open/ re-usable resources you can use when creating an OER (or for other educational purposes).