Step 1: Navigate to a slide where you are going to upload an image in a text box. From there, place your cursor within the text box and a word-processing toolbar will appear.
Make sure you create some space (enter+return key) to space out the image from the text that you are updating.
Step 2: Click the Insert/Change Image as highlighted above which will open up a new box where you will be able to determine which image you want to upload.
Step 3: Open up your image gallery and select which image you would like to upload. Once selected, you will be able to adjust the aspect ratio of this image as well.
Once uploaded, you can also resize the image by dragging it to corners to your liking. You can also use the word-processing bar to align the image as well.
*Tip to have the image resize with the window size
While setting the dimensions of the image (Width x Height) set the width as a percent, such as 80% and the height as auto. This will make the image 80% of the available width of the text area based on the size of your browser.
A width of 100% will make the image fill the entire width of the text box.