Collect your pics for slides into one folder. In Freemake slideshow maker click the '+Photo' button. Select all the photos from the folder. You can also simply drag-n-drop the library into the app.
If you want the pictures to be particularly organized, you can either name alphabetically or in numerical order or reorder them right in Freemake. To do it, double-click the slideshow in the app to open a built-in video editor. Click the colorful button (with blue and green) at the bottom of the editor to change the order of your images. Move the images in a necessary sequence within the slideshow project. Here you can also shuffle the pictures, delete them or add more visual files.
You can add pics of various extensions and resolution. Keep in mind that by default the software changes the resolution to that of the smallest photo in the library to prevent quality loss. If you need, you can select a bigger frame size for your presentation. However, the borders will be added as a frame to small images.