Hi all,
Is it possible to design a book cover in LibreOffice without using Gimp or Photoshop?
Are you asking about book covers for eBooks? Meaning that if you export a document to e.g. EPUB (I use a plugin for that), the specified page is used for the cover image?
It is certainly possible, but it depends greatly on the specific requirements. If you require a fairly plain cover (e.g., text over a coloured background), then Writer is more than capable. A moderately detailed design (e.g., involving graphic elements), while possibly able to be created in Writer, is probably best created using an external tool and imported. A complex design (e.g., involving elaborate vector use, raster manipulation, or some combination thereof) is certainly best created externally and imported. Note that GIMP / Photoshop are applications for raster manipulation.
There is any video or website explaining how to do it in LO Writer?
Not in general terms, because as my answer suggests, a cover can vary greatly in terms of design. What are you wanting to do (in design terms)? The comment about by @bencomp about using a plugin to assist with ODF to EPUB conversion is probably an easier topic to find information on.