This tutorial will guide you in how to give your photos a HDR (high dynamic range) effect. You can read more about HDR here. Start by opening your image in Acorn, File ▸ Open. Adjust the levels by selecting Filter ▸ Color Adjustment ▸ Levels and clicking the auto levels button (keyboard shortcut ⇧⌘L).
Go back to your original image layer and duplicate it again, Layer ▸ Duplicate Layer. Move this layer to the top of your layers list, above the inverted layer. Change the blending mode of the layer to "Linear Dodge" and decrease the opacity to around 30%.
If any areas appear too bright, use the eraser tool on the first or second duplicated layers to darken them. Set the eraser tool opacity to around 15-20% to prevent any obvious lines.