I guess it's a basic thing but I just don't no how to do.
I have a bunch of jpgs here to animate in Aftereffects. As they are all diferent size I want to resize them in Photoshop. How can I apply 'Image > Image size' to more than one picture at the same time?
As you see I am not very familiar with Photoshop so I really appreciate some detailed 'howto'.
Perhaps, create an Action.
Open your first image in PS, than click the Create New Action button in the Actions palette. This will record all of your actions.
You can resize your image by pixels, percents, etc.
Save and close the image.
Click the Stop Playing/Recording button in the Actions palette.
Now you can drop all of your images into the PS icon and apply your new action to each of the opened images.
Or, now that you have defined an action, you can do a batch process via File/Automate/Batch.
Or, now that you have defined an action, you can create a Droplet on to which you can drag and drop a batch of images.
I suggest you note your actions on paper/.txt file as you develop them. It helps for learning how actions work and to understand the possibilities and limitations.