Typically, many software packages are installed via .dmg files. A .dmg file is a file which when launched, mounts a virtual volume on the desktop. The volume can be opened and applications copied out of it. The .dmg file can be customized to create a simple installer. Here's how.

Open Disk Utility and click the New Image button at the top. Name the image, and make sure you set it to a large enough size to hold all the files for your installation.

Find the mounted image on your desktop and Get Info. Next, create a new folder on the desktop and Get Info. Select, copy and paste the icon from the mounted image to the folder. This new folder icon will be used in Photoshop to create a custom disk image icon.

Go into Photoshop, create a brand new layer and open IconBuilder plugin. Make sure the platform is Mac and the file type is All. Click the Import button and choose the folder which recently had the new icon pasted on in the Get Info dialog. Next, press the Apply View to Photoshop Layer. This will copy this 128x128 icon to the new layer recently created.

Choose the application icon layer, then delete all the icons except the 128x128 version from it. Next, choose the Edit > Transform > Distort and distort the bounding box of the 128x128 application icon so that it fits perfectly on top of the volume icon. Press the Enter key to set it.

Composite the Application icon onto the Volume Disk icon by selecting the Layer > Merge Down menu. Then, run the Convert 128 to others Action to fill out all the icons.

Open IconBuilder plugin again. This time the platform should be Mac and the filetype is All. Press the Save Options button and choose Save As: Folder with custom icon. Press OK to save your settings. Then press the Save buton and name a folder for the icon to be saved.

Now, go to the folder just saved, and Get Info. The new icon will be displayed at the upper left. Next, Get Info for the recently mounted volume created earlier. Copy and paste the newly created icon to the mounted volume Get Info panel.

Now, the mounted volume has a custom icon associated with it.

Open the newly branded volume and copy your standalone files into it. Create a new folder named "bg" or "images" and copy the background.jpg you just saved into it. Your file structure should look like this picture.

With the window open, select menu View > As Icons. Next, select View > Show View Options.

Select the This window only radio button.
Set the icon size to 128 x 128.
Select the Background > Picture radio button and browse to the JPG file you created on the volume.

Next, select menu View > Hide Toolbar

Your window should look like this:

Now, center your application icon over the round rectangle area, and move the bg folder way below so it is not seen. Square up the window and close it. Double-click the volume to see if it opens correctly.