You can flip a picture either horizontally or vertically in two different ways within either Google Docs or Google Slides:

Within Google Docs:

1. First, find the image that you would like to flip, and copy it by either right clicking on it and selecting 'Copy', or highlighting it by clicking and dragging your cursor over it until it is a different color and typing "Ctrl" and "C" on your keyboard.
2. Next, Go to "Insert" then "Drawing", then "New" within the toolbar in your Google Doc. A new box will pop up that has a checkerboard pattern.
3. Here, paste your image by either right clicking and selecting "Paste", or by typing "Ctrl" and "V" on your keyboard. After the image is in the drawing space, click on "Actions" within the toolbar, then scroll down to "Rotate", and then select either "Flip Horizontally" or "Flip Vertically".
   
4. After the image is flipped, click "Save and Close" to return to your document.
   

Within Google Slides:

1. Again, first find the image you would like to flip and copy it using the same method as above.
2. Next, paste your image into the Slides presentation the same way as above.
3. With your mouse cursor, highlight the image if it isn't already, then click one of the small blue squares outlining the image and drag it the opposite direction. The image's outline should follow to where your cursor is. Once you're happy with the size, release your mouse button and your image should be flipped. You can do this to flip both horizontally and vertically.

This method is far faster and easier, and you can simply flip the image within Slides, copy the flipped image, and paste it in your Doc.