resize psd file for thumbnail layout

Thanks.  I am unfamiliar with page layout software but will look into it.

When I initially dd this (created a blank psd document and dropped jpg's into it), I got messages about color issues which, unfortunately, I selected to not show again so now I can't replicate them.  So the problem with not getting usable color was, I assume, a result of opening the jpg's in PS and dropping them into a psd document.  The messages were about displaying color properly.  Also, I'm not sure what you mean when you say "color in your mockup doesn't matter."  If you're grouping images according to color, then surely it does.

Thank you.  The jpg's were all exported from Photoshop (I have Version 19.1.1) using File>Export>Save for Web (Legacy) and "Convert to sRGB" was checked.  The images are all artworks and the colors are not wildly off - in fact just portions are - so perhaps I am just being overly sensitive but I need fairly exact representations of the images in order to properly group them.  But I assume in any event the problem is not that to which you refer as it seems like these all are sRGB.  Thanks again.

Thanks, but I am having lots of problem with this.  First, I created a new psd for image layout with the dimensions 15"x50".  No real reason for that, other than if printed (which it won't be - this is purely for my review onscreen) I need about 3 thumbnails across and many rows. 

Then, I dropped one of the psd images into the layout document.  This created a new layer.  I went to Image Size in the layer and selected 4" for width; the height did not automatically populate so I put in 3" and this made the entire document very tiny on my screen - both background and image.  How can I manipulate the size of the image in each layer and move it around on the background without the background changing size?  Thank you.