There are several software out there which are designed to manage your collection of photos and images. However, Photoshop provides the best features in terms of flexibility and editing capabilities. Furthermore it can also preserve the original contents.

A much easier way for online printing of pictures can be achieved by the print poster collage. This is because it can be easily managed, edited as well as arranged in categories. These are usually given as gifts. Photoshop is however, still favoured over print.net.

In order to get your photos ready for printing, you should simply collect all your photos to be used in print poster collage in a single folder.
When dealing with scanned pictures, it is advisable to set the scanner resolution to maximum to obtain a clear output. You can start working by opening the photo editing software and creating a new file.

The first thing to do is to enter the width, height and resolution/DPI for your poster.     

It is important to make a note of the value of the resolution that you set as it may become useful as you go along. Usually, a resolution of 150 DPI or above is recommended.

When transferring images from Photoshop to the poster print collage, it is essential to have an idea of the quality of the image. The image must also be placed without disrupting the standards. To solve these problems, many tools are available. The cropping tool can be used for managing the image size with much additional functionalities available in Photoshop.

When you are done, you may save the file in either .jpg or .png format. 
The next step is to find a suitable company to process your photos online.
The poster printing company will take it from there to work on the final product.