Sendible is not making any changes to your images, any changes you might experience are coming from different settings on social sites and the web.
My image looks compressed and pixelated
Social sites, especially Facebook, prioritise fast loading images over high quality ones, which is why you might experience that your images look compressed after being posted to your social account.
Facebook will automatically resize and format your photos, so they’ll display properly on their site and in the apps.
For example, if you post a single photo, that photo will show on your News Feed with a resolution of 476 x 714 pixels regardless of its original size. Unless you upload your photo at that exact resolution, Facebook needs to do some scaling. However if you do upload your photo at that resolution, it’s going to look awful if someone clicks on it to see it zoomed in.
Facebook recommends three width sizes: 720 pixels, 960 pixels, and 2048 pixels wide. For photos the ideal size would be 2048 px wide.
Please check this article for recommended image sizes for each social network.
Facebook compresses the highest quality versions of your images to just under 500 KB. For this reason, rather than compress images yourself, we recommend uploading the highest quality JPEG you can, up to 4 MB in size.
For the highest quality cover photo, you want to upload an image that is 851 x 315 pixels and that’s less than 100 KB.
If you upload a cover image that’s less than 100 KB in size, it won’t be compressed at all.
The colours look different on my image
The reason why the colour of the image can change is due to having a colour profile set for your image that is not for internet use (CMYK).
CMYK refers to the primary colours of pigment: Cyan, Magenta, Yellow, and Black, that are mainly used for printing.
While RGB refers to the primary colours of light, Red, Green and Blue, that are used in monitors, television screens, digital cameras and scanners.
Hence if you're trying to upload a CMYK image online (and through Sendible as well), the photo might display in different colours, as CMYK is not supported by web applications.
Because of this the correct colour profile to use is RGB or sRGB, if you would like your images to display the colours as intended across all computers.
You can check what colour profile a photo is using by viewing its metadata.