I have heard of jQuery but I'm not familiar with it.
One listed is based on Bootstrap but LuxCal doesn't use Bootstrap so I don't know how or if it would work.
Before you decide to add in a WYSIWYG editor. People will want the ease of use of MS Word and sites like Facebook, Twitter etc. If you give the ability to insert an image they will want to be able link to an image or upload it from their computer. If the image is too big they will want it resized automatically. They don't want an error message that the image is too large. Most users don't know how to resize an image or how large the image they are trying to upload to know if it is too big.
If a user can include an image they will likely want to include or upload a video.
What you have done with LuxCal since I started using it is remarkable. If you feel adding a WYSIWYG editor to LuxCal fits your vision of what LuxCal should be and will be well utilized then I know you will take on that challenge as you have with the other features you have added that have been a challenge. If you feel that there are other features that are a better fit with LuxCal and utilized by users then perhaps your efforts and talents into those features are the route to take.