Step 1
Open the template in Adobe Photoshop and your picture side-by-side.
Step 2
Drag the layer with your custom image to the template with left button of the mouse.
Step 3
Click on the "eye transparency" of the previous image layer in your template's Layers Palette. This will make previous image transparent.
Step 4
Go to Edit > Free Transform (shortcut keys: CTRL/Command + T) to resize your image.
Drag the edges of the image until you are satisfied with the results.
Click "Enter" to apply the results.
And we're are done! My Congratulations!
You can also check video tutorial on how to insert images into Photoshop templates.
Useful resources:
Free printable wedding invitations in PDF format
Tips on printing invitation templates
How-to Tutorials
How to insert your picture instead of sample one into the template (video tutorial)
How to choose the right font for your invitation