For this example I'll be using a photo from my recent Shelburne Museum trip. It is a shot of two animal sculptures. I initially thought they were dogs (and they may be) but now I notice they have prominent whiskers and pointy ears. OTOH they have pointy noses too so maybe they are dogs. In any case the first step is to resize them to whatever you want for the web. Most photos I post here are 800 pixels on the long side but for this exercise I've gone with 500 pixels by 400 pixels to fit the space better. Be sure to sharpen the image after resizing. Every time you resize you lose sharpness.
Now add a border using Image>Canvas Size... The width you add now will represent the beveled edge of the mat. I generally add 12 pixels horizontally and 12 vertically for 6 pixels per side. Be sure the box by "Relative" is checked in the canvas size dialog. In the example below I've added a slightly grey border because I'm making a white mat.
* I later changed the background color of the blog to a middle grey so this is no longer evident but it is something to remember if you will be placing your image against a background of the same tone & hue that you chose for the pseudo-mat.
That is your basic pseudo-mat. You can fancy it up if you want by adding a texture. Set your magic selection tool to "0" tolerance and select the surface of your mat then apply a texture with the texture filter. I often use the canvas or sandstone textures. Keep it subtle though, very low relief and scale about 50%. You can also chose to do a pseudo-metal frame like my recent postings have. It is nothing more than another Image>Canvas Size... using black for the color and add around 40-50 pixels to the canvas. So there you go. Have fun. You don't even have to change blades in the mat cutter to get a clean cut.
Final image with sandstone texture (50% scale - relief 1) and a 40 pixel "black frame".