(Go here for a better reading experience (links, pictures): https://banjoalfred.medium.com/rarely-talked-about-useful-image-video-tools-for-social-media-managers-83e3bc06997d)

Remix by Buffer:

Transform any link, tweet, or product into beautiful Instagram content.

Removebg:

Remove image background automatically in 5 seconds with zero clicks.

Icons8:

Download design elements for free: icons, photos, vector illustrations, and music for your videos.

Pfpmaker:

Create an impressive profile picture in a few clicks.

Duotone:

Find beautiful free duotone images.

Gifski:

Convert video frames to GIF animations.

Stencil:

Easy-to-use graphic design tool.

Infogram:

An intuitive visualisation tool to create engaging infographics and reports in minutes.

Fastory:

Create branded content for Instagram, Snapchat, Twitter, and Facebook stories. Fastory has a robust menu of content options including:
Games
Quizzes
Articles
Videos
Chatbots
Surveys

Apple Clips:

Apple Clips is a video editing tool that lets users create and share short videos complete with special effects, text, and graphics.

Wordswag:

Generate cool texts, words & quotes on your photos.

Skitch:

Skitch is a screenshot tool with easy annotation features. You can circle things, point to things, blur things, and add text with a few clicks.

Placeit:

Put your website or app inside cool stock photos. You can create beautiful images that features your home page, blog, app, or service.
Choose a background from Placeit’s library. Upload a screenshot. Placeit does the rest.

Page2Images:

Page 2 images is a full-page website grabber.
See a web design that you like? Use Page 2 images to grab the whole thing in one complete shot.

Pictalio:

Create amateur portrait videos.

P.S. If you like content like this, follow me on Twitter @banjoalfred