I am creating a practical guide book with images. I am currently using Scrivener which is writing software for the edit stage. I need to include images into the text, but I will then pass over my draft to a layout designer to layout out the project in Indesign. Should I bother putting full size images into Scrivener, or should I just put placeholders in and then hand over the full size images to the designer?
Use placeholder images in the text. Then deliver full size images with the text to the designer.
Some applications can down-sample placed images. I don't know whether Scrivener does. It is generally safer to always send full size images separate from any text. This avoids any possible resampling issues.
As a user of both programs, I would definitely say just put in a placeholder and let your designer import the images. Having them in the Scrivener text isn't going to help. You export them as Word or whatever, and then as a designer I'm going to strip out everything but the raw text so I can style and format it properly in InDesign. I'd much rather you put [PLACEHOLDER] so I can find it easily.
If you want to include instructions about size, text wrap, alignment, etc. by all means do so, but don't bother sticking the image into your Scrivener document.
(and yes... this is the same answer I gave on Writers to the same question you asked there. ;) )