Welcome to the forum. We were all beginners so don't hesitate to ask questions.
There are a number of ways to do this. Rather than type in a step by step look at this tutorial. If this does not work do a search for create mat photoshop.
Basically you want to convert your photo from a background layer (turn on layers in Window) to a layer. (rght click on layer) so you can resize on layer.
Move the colored layer below the image layer (drag) or else the image will be a solid color.
Hope this gets you started.
GeoAtl, from your message I understand that you have an image with the size of 16 x 20 inches as indicated in the Image Size dialog or in the Info panel when you select All (Ctrl +A). And you also have a mat layer and an image layer on top that you want to be 13 x 17 inches.
Here's one way.
Make sure the desired layer is selected in the Layers panel.
Pick a selection tool, like the Rectangular Marquee Tool (M), select all (Ctrl + A), right click on the image, and choose Transform selection. In the Transform Selection options bar, which is under the main menu, for W and H, type 13 in and 17 in. If the units are not already in inches, make sure to type the "in" after the number or first right click in the field and choose inches for the unit. Press Enter until the transform selection box disappears and you have only the selection
In the Layers panel, click the third button "Add Layer Mask" at the bottom of the panel - alternatively you can also select from the Layer menu Mask > Reveal Selection.
In the Layers panel, click the chain symbol between the icons representing the layer and the Mask. This will allow you to move the layer and its mask independently from each other.
To move the image the way you like to fit in the frame (mask), make sure that the focus is on the Layer icon and not its mask by clicking on the layer's icon in the Layers panel. The white border around the icon of the layer or its mask indicates which one will be edited (moved in your case)
Select the Move tool (V) and move your image until it fits best in the frame (mask)
If you want to center the framed image in the mat, with the image layer selected, hold Ctrl key down and click on the mat layer to add it in the layers selection, make sure the Move tool is selected (V), and in the tool options bar under the main menu, click the alignment buttons representing horizontal and vertical alignment.
Thanks. I’m getting closer, having prob with sizing the picture (and mask) to 13x17 inches, so it’s in center of the 16x20 canvas. Here’s what’s happening:
Have 2 layers:
Layer1- Background 16”x20”
Layer2- SunsetPicture. Placed this picture here, it sizes itself initially to about say 15x20. Need to size it to 13x17, will also have to crop one edge (using mask I think is the approach) due to aspect ratio
Now trying to size the sunset picture down and move it around to let the background (or another layer) be the fake mat…
Clicked sunset picture layer, did Rectang Marquis Tool per ‘emil emil’ suggestions… Cntl A, rt click Transform selection
But here’s what seeing and the part not understanding…
1) INFO panel on bottom right shows correct canvas overall sizing of
2) Transform Selection Options bar (under main menu) shows “odd” seemingly unrelated size of
W8inxH10in W100% H100% etc. (<unsure why different than any of above)
When manually try to enter into
Transform Selection Options bar…
W13in x W17in… the box jumps right and down, off the canvas. Then press enter twice, and
the INFO panel now shows the final rectangle of
W11 x W13 (another set of odd dimensions, plus when I create mask, select it, and move the mask box around, it does mask the picture, but wrong size, it seems to be a too-large mask, not 13x17 but maybe closer to 16x20
Greatly appreciate any suggestions/insights to clarify this, or some newbie step-by-steps to create the fake mask in original post (have tried all replies so far). Apologies for not being up to speed yet on basics like this.