My understanding of pixel dimensions vs DPI vs image size

Hi Everyone!
I saw a thread on the Etsy forum about printing/dpi/image size and was inspired to share my knowledge! I know all that jazz can be super confusing so here is my attempt to simplify and explain in layman's terms.

-- DPI vs Image Size --

dpi (dots per inch) -  Think of it like, the more dpi you have, the more color information and data your computer has about your image. So more dpi = more data = more quality

I think what is confusing is that when you change the dpi you are also adjusting the pixel dimensions, which is not necessarily the image size per say. To make things simpler think of pixel dimensions as the quality of the image NOT the size of the images. Personally, I ignore pixel dimensions unless I have absolutely have to follow specific pixel dimensions for web images.

Image size - You dictate the size when you create the file (inches, pixels, feet..etc.)
Just because you have an image that is 5x7 does NOT mean it will print well.
If you have a 5x7 image with 300 dpi, it will probably print great at 5x7!
If you have a 5x7 image with 72 dpi, that is ok for web but will probably print blurry/pixel-ated. 
(72 dpi is ok for web, NOT for print)

Take a look at the image below.
Same image, same size, different dpi, different pixel dimensions. Take a closer look at the edges and you can see the left one is pixel-ated and the right is much clearer.

 -- What does it all mean?! --

If you take a 5x7 image at say, a respectable 150 dpi, it will probably look nice at 5x7, but if you try to stretch it to a 8.5x11 page it will get pixel-ated. It just doesn't have enough data to stretch itself out to a larger size and still look good.

It's like you have a ball of play dough (dpi) and it makes a nice 5x7 square with a 1inch thickness (image size). If you try to stretch your play dough to be 8.5x11 it will get thinner and maybe even have holes in the middle since there is not enough dough (dpi) to stretch that far. You will have a larger piece (image) but it will be stretched out and thin (you lose quality)

You can change your dpi/sizes/dimensions after you work on an image but if your working with a rasterized image (raster is pixels) then just changing the numbers will not make the quality better. If you change the numbers then you would just be stretching your play dough making it thinner, to compensate for the change in size. It's best to set up your image size and dpi BEFORE you start working on your images.

When in doubt make it a higher dpi and you can always take it down later, but keep in mind...
- More dpi means a larger file size.
    Too big and it could slow down your computer
- You don't want to send your customer a huge-mungus file that will take them ages to download!
       so work high, but before you send it to anyone make your dpi a manageable size while still retaining quality.
 
In general here are a few rules I follow....
- Work in 300 dpi or higher 
    Especially if you think you might print it later

- Scan images in 300 dpi or higher 
  The smaller the image the higher dpi I will scan it in just in case I want to try to print larger later

- 72 dpi is cool for WEB ONLY
   But I always work at a higher dpi and take the dpi down when I save the jpeg to post online

-  Scan images as TIFF 
   I have heard they are better for compression reason. Compression means you lose image data so that you can have a smaller file size and save space. JPEG is compressed so no matter what you do when you save as a JPEG you will lose some quality (though it may not be noticeable unless you try to enlarge your image)

- I make a PDF if I'm going to a printer.
   It preserves the layout, so if you need to be super specific with your layout or size on a page you can do it with PDF, but make sure the printer DOES NOT CHECK THE FIT TO PAGE BOX, that will screw up your layout and size, I speak from experienc *tears*
 Also opening a pdf in acrobat will give you different print options, but really JPG is probably more widely used. I feel like PDF is obsolete unless your making a business document or pamphlet.

and finally ....

- ALWAYS TEST PRINT 
- ALWAYS HAVE SOMEONE ELSE LOOK AT YOUR TEST PRINT
   because you have been looking at your image for so long, even if there is an obvious mistake, trust me, you wont' see it. It's always always helpful to have a second set of eyes on your work before you send it out to a customer. 

I hope this is helpful!
Also, if you notice any info that is inaccurate, please let me know. I am by no means a pro at this so any info sharing and corrections would be super welcome! =D

Thanks!