@Manoj Dega:

  The only filters you can apply on id fields (in this case parentId by which you can say if it is an attachment for Account) is "IN" or " =" In the other words you can not filter on the attachments directly to fetch only account related ones., if instead you can modify your structure in such a way that "Deleting Attachments That Belongs To Particular Accounts" you can query attachments using "IN" operator., For eg.,

Set<id> AccountIdsForAttachmentDel=new Set<Id>();
//Loop through all your accounts for which attachments needs to be delted and collect the ids

then you can query

List<Attachment> AttachmentsForDeletion=[select id from Attachment where parentId IN : AccountIdsForAttachmentDel];

delete AttachmentsForDeletion;

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Worst Case you need to start from Attachment, you might end up this way: 

For(Attachment AttachmentRecord:[select id,parentId from Attachment limit 50000]){
    if(String.valueof(AttachmentRecord.id).StartsWith('001'))//check if the prefix of id is 001 which determines if the record belongs to Account or not
{
//Code for your criteria
}
}

Note: If you have attachments more than 50K you need to have a logic that says  fetch remaining attachments and get those belongs to account to process them

Thanks,
Balaji