Canva's stock images come from Pixabay, Pexels or CC0. These sources are covered by the Free Content License, which does not require attribution (i.e. a citation)
If you pay for Canva and therefore get access to the paid images, you have non-exclusive, perpetual rights to use the images, so you do not have to cite them.
All MS Office products have three image options:
If you are using the embedded search or uploading your own images (points 1 and 3), then you are responsible for checking and including a citation if necessary.
If you are using the stock images from PPT, you are not required to give attribution when using them in MS office based documents and software, but again,citation will never hurt you. Source information reminds you where that image came from when you review your presentation, or if somebody asks.
Providing source information tends to be a helpful practice and habit. Sourcing doesn't have to be as formal as a full citation but indicating source information such as "image source MS office" will let individuals viewing your presentation know where the images came from, and stand as a record for you as to where you pulled the images from.