Hello and Good day to you! Try this :
Open Photoshop, don't create a new file or something.
drag and drop the photo file you need to edit then.. It would create the image in a background layer and you'll notice that it's locked.
What you need to do is convert it into a layer by right clicking that layer in the layer section and select "Layer from Background"..
After that, select the select draw box tool from the left handside toolbar. then, right click on the image and select "Free Transform".
This should make it possible for the image to be rotated, re-sized properly by hand or by inputting precise digits.
HOW TO FLIP the image : while the picture is on free transform, right click on it and you can then select to flip it horizontally or vertically.
After your happy with the new edit, select on the draw box tool again. You'll be prompted if you'd like to apply the Transformation. click "Apply".
If you need the background to be white, or black just create another layer underneath the first layer you created and fill paint it with white or black or even any color you like.
HERE IS ANOTHER thing you can try : If you want the images to have the exact sizes all the time, you can adjust the canvas size by selecting "image" from the menu bar and selecting "Canvas Size.." you can then input the exact canvas size you would want it to be by pixels or other measurements. this should also make the whole picture area resizable without affecting the "main image".