PREFACE: This may not work with every single browser, but it was discovered and tested using Pale Moon, a fork of Firefox.
Some websites have instituted a system which makes it difficult for users to save certain images, disabling the ability to simply "Save image as" or even "View image" through some sneaky maneuvering on their part. Rather than getting the image by itself in the browser tab, the site redirects to a page where the image is visible but seems to exist in an unsavable state right-clicking it is no different from clicking on an empty space.
Fortunately, you can be sneaky as well. The demonstration here applies to Tumblr, but it can be used on any site, so long as you can grab the full-size URL of the image. (This is for cases where pasting the URL directly into the address bar doesn't do the job, either.)
First, you'll want to find the URL of the image. Right-click somewhere on the page and choose "Inspect element" to go into the page's source code and poke around. You can find your way to the image by thumbing through the code and opening up all the appropriate tags one by one, or by entering "jpg" or "png" or whatever type of file the image is in the search bar. Eventually, you'll come to something that looks a lot like this.
Double-click on that image's URL and it should expand into something like this.
You'll notice the resolution of each image is within its URL. Choose the one you want, (probably the largest,) and highlight it as shown.
Copy that URL. Now the fun begins.
Maximize your browser window. If it's not large enough, things could end up getting thrown out of proportion. Then go to some other site in another tab, pick out an image, right-click it and choose "View image". Once you're viewing the image alone, right-click on it and select "Inspect element".
Now, see that underlined image source URL in the quotes? Right-click it and choose "Edit HTML", then paste in the URL of the other image which you copied before in its place, right between the quotes. Click somewhere else in this region of the page to cement your change and behold! The desired image is now in the browser window! Right-click and save it,... then convert it to a .png in IrfanView to preserve its quality for the ages, if need be!