Putting big photos in your website you can hurt your speed. You should upload medium images. The image optimization you can succeed it with WordPress plugins. I suggest you to use 2 free plugins WP Smush and Compress JPEG & PNG images with TinyPNG API account.
Reduce the Size of Your Images
The size of the image is very important for the speed of your website. I recommend you 2 steps. The first is the reducing before uploading the image. WordPress allows you to crop images. A popular size for the images is 800px wide. Check the needs of your template and especially your slider and set up the right dimensions in your images. You can check the themes documents for optimum image sizes. The second step is the setting of the plugin. You can use a plugin like Compress JPEG & PNG images with TinyPNG API account. The Tiny PNG reduces the size of the images without any loss of quality. Uploading the appropriate resolution images to your website, you’re take care your image SEO.
Use Descriptive File Names
Your image file name should be as descriptive as possible. A trick to save time is to name the image before you upload it to your website. WordPress keeps your image file name as the Title of your image when you upload it to your website.
So your file name should not be IMG001.jpg.
The title should have some relevant keywords and hyphens between the words, not underscores. The reason of doing that is in order to help Google to use the file name to give search engine robots any indication of what that image is showing.
Run the Image Through an Optimizer
Can the compression be harmful?
If you select the right tool for compression there will be no quality difference in the photo. So the appropriate photo optimizer plugins strip the photo of unnecessary information like camera details, metadata etc.
You can use WP Smush It or Compress JPEG & PNG images to automatically compress photos.
Add Unique ALT Text
The ALT (Alternative) Text is crucial for your photos. Google ALT text that ALT text is counting and can not missing. When uploading your photos in your website you should set the ALT text with keywords. However, not exaggerate with the use of keywords Google can flag you if finds out excessive use.
Make Sure Your Image is Relevant
All the in your post should be relevant. So, keywords, tags and photos should be relevant. Search engines do not like to show irrelevant information, so if they find out that you cannot obey the rules they can flag you. Also, users do not like to be confused so keep the images relevant to the content.
You can see more SEO optimization tips here 9 quick SEO hacks for the SEO newbie.